General teachings for the open-circuit liquid cooling of gas turbine vanes are set forth in U.S. Pat. No. 3,446,481 -- Kydd: U.S. Pat. No. 3,619,076 -- Kydd; U.S. Pat. No. 3,658,439 -- Kydd; U.S. Pat. No. 3,816,022 -- Day; and U.S. Pat. No. 3,856,433 -- Grondahl et al., for example. In these patents, the cooling of the vanes, or buckets, is accomplished by means of a large number of spanwise-extending subsurface cooling passages.
The invention described and claimed herein is applicable in those constructions of liquid cooled buckets wherein the coolant passages are cylindrical in configuration. Thus, for example, preformed tubes employed as coolant passages preferably form a setting for the use of the instant invention. However, the concept of employing preformed tubes as subsurface coolant passages in turbine buckets, per se, as well as particular arrangements for incorporating such tubes in the bucket construction are the invention of other(s). Thus, the use of preformed tubes set in a copper matrix is shown in U.S. patent application Ser. No. 749,719 -- Anderson, filed Dec. 13, 1976, and assigned to the assignee of the instant invention.
Tests made on open-circuit water cooled buckets with the axis of each coolant passage oriented approximately perpendicular to the turbine axis of rotation have established that under preferred conditions of operation (e.g., rate of water input, rotating speed, temperature of motive fluid, etc.) the water travels in a thin film through each passage. The water film is pulled through each channel by centrifugal force, achieving high radial velocity. At the same time, the film experiences a strong Coriolis force, which, at operational rates of cooling water supply, pushes the film into a limited area extending along the length of the coolant passage disposed the most rearwardly as the coolant passage is rotated.
When this occurs, the liquid film covers but a small fraction of the surface area of the coolant passage and the cooling capacity of the liquid flow is reduced. For a given heat flow into each coolant passage, or channel, this limited cooling area results in a higher coolant channel surface temperature and this in turn results in a higher bucket skin temperature and shortened bucket life. It would be most desirable to increase the effective cooling area within each coolant passage at any given rate of liquid coolant flow whereby the bucket skin temperature can be reduced and the cycle fatigue life extended.
The invention described and claimed in U.S. patent applications Ser. No. 743,272 -- Kydd, filed Nov. 19, 1976, and Ser. No. 743,271 -- Dakin et al., filed Nov. 19, 1976 (both assigned to the assignee of the instant invention) are directed to this same problem. In the Kydd application means (e.g., raised or recessed helical configurations) are provided within individual coolant passages for providing a swirling motion to the liquid coolant. In this manner the liquid coolant is subjected during operation to a first centrifugal force acting in the radial direction, the Coriolis force and a second centrifugal force acting about an axis extending in the general direction taken by the coolant passage.
In the Dakin et al. application, cylindrically-shaped coolant passages for liquid-cooled turbine buckets are converted into at least two helical sub-passageways by flow splitting means introduced into individual coolant passages and fixed in place as by brazing or tight mechanical fit. In addition each flow splitting, or flow modifying, means is provided with means disposed therealong for interrupting the liquid flow in each helical sub-passageway.
Various vortex flow promoters in single phase stationary systems have been described in an article by A. E. Bergles in Progress in Heat and Mass Transfer, Volume I, Edited by V. Grigull and E. Hahne [Pergamon Press, 1969]. In stationary systems the cooling fluid is forced through a channel by a pressure drop and the vortex promotion is accomplished at the expense of increased pump power. No discussion or guidance is provided therein of any solution to the problem of increasing the effective cooling area within coolant passages in a rotating system.